1.) A guy with a hugely over-inflated ego, who in an attempt to get un-due attention for himself, will act like a jackass, because, in his deluded state, he will think it's going to make him look cool, or make others want to be like him. The person may even insincerely apologize later on, but only in an attempt to get more attention, or to excuse his blatantly intentional, and unrepentantly tool-ish behavior.
2.) Someone whose ego FAR exceeds his talent, intelligence, and likeability. But, of course, he is clueless regarding that fact. He erroneously thinks he is THE MAN!
3.) Someone who others normally refer to as a prick, dick, or schmuck.
4.) Someone who acts like a dick, because...well...he's compensating.
5.) Kanye West
Observer #1: Hey, did you see Kanye West's idiotic stunt at the awards show?
Observer #2: Oh, you mean, "Mr. It's-All-About Me?"
Observer #1: Yeah...what an asshole.
Observer #2: I know. He's such a tool!